The present invention relates to a reproducing apparatus for optical information recording media and, in particular, to a reproducing apparatus for optical information recording media for reproducing by an optical head signals beforehand recorded on optical information recording media in a mark edge recording method.
1. Description of the Related Art
When recording information on optical information recording media from which information recorded thereon (represented by beforehand recorded signals) is reproduced by optical means, there is employed a method called a mark position recording method for the recording media of the standardized first generation as shown in FIG. 1.
In the mark position recording method, intensity of a recording laser beam is modulated by non-return-to-zero (NRZ) signals ((B) in FIG. 1). corresponding to codes of series Y ((A) in FIG. 1) attained by modulating the recorded information so as to form record marks on the recording media as shown by (c) in FIG. 1.
In the reproduction of the signals thus recorded on the recording media, a laser beam is illuminated thereonto by an optical head to sense changes in the light intensity or polarization plane of light reflected on the media. The changes are then converted from optical information into electrical information to reproduce a signal R of (D) in FIG. 1. In the reproducing apparatus, the reproduced signal R is differentiated to obtain a signal shown by (E) in FIG. 1 so as to produce a mark position sense signal shown by (F) in FIG. 1.
In contrast therewith, there has been heretofore known a recording method which improves the recording density and which is called a mark edge recording method as shown in FIG. 2.
In the mark edge recording method, codes of series Y ((A) in FIG. 2) obtained by modulating the recorded information is subjected to an NRZI modulation to create a pulse series Z of (B) in FIG. 2. Light intensity of the recording laser beam is modulated by the pulse series Z to from on the optical information recording media recording marks in which the leading an trailing edges each correspond to code "1" as shown by (c) in FIG. 2.
When reproducing the signals thus recorded on the media, a laser beam is illuminated thereonto by an optical head to detect changes in the light intensity or polarization plane of light reflected on the media. The changes are then subjected to an opto-electric conversion to generate a reproduced signal R of (D) in FIG. 2. In the reproducing apparatus, the reproduced signal R is compared with a threshold level I to produce a detection signal including a pulse series Z shown by (E) in FIG. 2.
In accordance with the mark edge recording method, since the leading and trailing edges of a record mark each correspond to "1" in series Y, the recording density is approximately doubled for the same recording mark length when compared with the mark position recording method in which a record mark is created for each "1" of series Y.
However, in the reproducing apparatus of the mark position recording method, the record mark positions are detected according to differential signals and hence the apparatus is stable against changes in a direct-current (dc) component of the reproduced signal. In contrast therewith, in the reproducing apparatus operating in the mark edge recording method, the record marks are sensed by achieving the level comparison between the reproduced signals and the threshold level as shown by (D) in FIG. 2. Consequently, there is increased the chance of read errors due to the dc fluctuation in the reproduced signal.
In consequence, when achieving a signal recording and reproduction in the mark edge recording method suitable for a higher recording density by an optical head, it is essential to set a threshold level for the binarization of reproduced signals. In the optical information recording media of the first generation, there occasionally exists media which is unavailable because an ideal record pit contour cannot be formed due to a characteristic thereof. However, thanks to the recent improvement of the media and the record compensation technology, the edge shift has been reduced.
However, when an alternating-current (ac) amplifier is adopted in place of an expensive low-noise wide-band dc amplifier, since there may be missing dc-free codes to be recorded with a high density, the dc fluctuation becomes conspicuous in the leading portions of sectors constituting a code file. Moreover, when using such media having birefringence as a polycarbonate substrate, there appears a dc fluctuation up to about 10 kHz relative to the direct current in the circumferential direction of the disk, resulting in increase of data read errors. As above, it is to be appreciated that the data read errors may frequently take place due to the dc fluctuation.
FIG. 3 shows in a block diagram an example of the conventional reproducing apparatus for optical information recording media provided with a countermeasure against the dc fluctuation. In the diagram, a reproduced signal obtained from the media by the optical head is directly supplied via an input terminal 30 to a comparator 32. On the other hand, the signal is fed to a low-pass filter (LPF) 31 so as to extract a dc component from the signal. The dc component is then fed to the comparator 32.
In the comparator 32, the dc component outputted from the low-pass filter 31 is used as a threshold level for the level comparison of a reproduced signal inputted thereto. A series of binarized pulses obtained according to the result of comparison is outputted as a sense signal to an output terminal 33. Resultantly, there is attained from the conventional apparatus a sense signal from which the influence of the dc fluctuation has been removed.
FIG. 4 is a block diagram showing another example of the conventional reproducing apparatus for optical information recording media provided with a countermeasure against the dc fluctuation. In the diagram, a reproduced signal obtained from the media by the optical head is fed via an input terminal 30 to an amplifier to be amplified therein. The amplified signal is then delivered to a mean value circuit 37. From the circuit 37, a signal corresponding to a mean value of the maximum and minimum values of the reproduced signals inputted to the apparatus is fed to a sample and hold (S/H) circuit 38. Moreover, the reproduced signal outputted from the amplifier 36 is directly delivered to a comparator 32.
The sample and hold circuit 38 samples, according to a preamble gate signal inputted via an input terminal 35 thereto, a signal outputted from the mean value circuit 37 during a preamble field reproducing period of the media. In other field reproducing period, the signal sampled and held therein is supplied as a threshold level to the comparator 32. In the comparator 32, the threshold level is compared with the reproduced signal so as to output a detection signal corresponding to the dc fluctuation to an output terminal 33.
According to the conventional apparatus described above, the threshold level also follows with respect to time as indicated by II the reproduction signal having an envelope with an dc fluctuation shown by (B) in FIG. 5, thereby absorbing the dc fluctuation. In this regard, (A) in FIG. 5 shows an eye pattern of a reproduced signal in which the dc fluctuation is missing.
However, as can be seen from the pattern of the reproduced signal of FIG. 14, the resolution is decreased due to the high-density recording. Additionally, in the reproduction of a particular pattern with a dc component, since the threshold level is obtained by integrating the reproduced waveform in the conventional reproducing apparatus of FIG. 3, the threshold level is shifted downward from the optimal value as indicated by a dotted line III in FIG. 14 and hence the error occurrence probability is increased. Furthermore, in the conventional reproducing apparatus of FIG. 4, the threshold level, which is the mean value of the maximum and minimum values, is obtained as denoted by a dotted line III in FIG. 14, namely, below the optimal value, and hence it is impossible to carry out a satisfactory reproducing operation.
In addition, in case where information is recorded according to the direction of magnetization of a magnetic film of the optical information recording media, when the film of which the direction of magnetization is beforehand fixedly aligned in the information recording operation is applied to a recording method in which the film is locally heated by a laser beam according to information to control the direction of magnetization to a direction (opposite to that beforehand aligned) corresponding to a bias magnetic field externally applied to the media, the length of each record mark for record data varies depending, for example, on the change in power of the light source in the information recording operation using the optical modulation and on the change in environmental conditions as shown in FIG. 15.
Namely, in (A) of FIG. 15 showing the case of an insufficient recording power, the length P1 of a record mark becomes shorter when compared with record data. In (B) of FIG. 15 presenting the case of an optimal recording power, the length P2 of a record mark appropriately corresponds to the record data. Furthermore, in (C) of FIG. 15 designating a case of an excessive recording power, the record mark length P3 is longer than the record data.
In the conventional apparatus, also for any information recording media in which record marks are formed, the position of patterns respectively corresponding to the maximum recording frequencies are shifted upward or downward and hence the eye pattern symmetry of reproduced signals is destroyed as shown by (A) to (C) in FIG. 15. However, the threshold level designated by a dotted line in (A) to (C) of FIG. 15 is in the vicinity of the central point between the maximum and minimum values of reproduced signals in either cases. Therefore, particularly, in the conventional apparatus shown in FIG. 4, the threshold level is shifted from the optimal threshold level, leading to a problem of increase in the error rate in the information reproduction.